This invention relates to heat fixing systems, and more particularly, to improvements in fuser apparatus and methods for fixing particulate material such as resinous toner particles, that are used in electrostatic copiers.
More specifically, the invention relates to an improved system and device for extending the release life of heated roll or belt fusing devices. Although the invention is considered to have general application, it is particularly useful in the field of xerography and has an important application in the fusing of resinous powder images produced by electrophotography or xerography onto substrate sheets, e.g., paper and the like, to which the powder images have been transferred after they have been formed by deposition of powder on an electrostatic latent image. Therefore, for convenience of illustration, the invention is described with reference to its use as a heat fuser for xerographic powdered images.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from the member to a substrate such as plain paper, with subsequent affixing of the image thereto.
In order to affix or fuse electroscopic toner material permanently onto a photosensitive member or other substrate by heat, it is often necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member or substrate. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is a conventional and well-known technique.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is either externally or internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered, is moved through the nip formed between the rolls with the toner image contacting the fuser to affect thereby heating of the toner images within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the toner whereat the toner particles in the image areas of the toner would liquify and cause a sheering action of the molten toner resulting in "hot offset." By the provision of the proper roll surface material, offset of toner particles is minimized
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof, i.e., "cold offsetting," by imperfections in the properties of the entire surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the surface of the backup roll during periods of time when no substrate material is in the nip and before the backup roll can be moved out of contact with the fuser roll. Moreover, toner particles can be picked up by the fuser roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
To prevent such toner particles from being transferred to the copy substrate, it is necessary to remove the toner particles from the fuser roll. It will be appreciated that if enough toner accumulates on the fuser roll, the copy substrate, for example, paper, fed through the fusing system will be adversely effected.
One arrangement for minimizing the foregoing problems, including that which is commonly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene commonly known as Teflon, to which a release agent such as silicone oil, is applied. Silicone-based oils, which possess a relatively low surface energy, have been found to be a material that is suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form thereby an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from adhering or offsetting to the fuser roll surface.
Another arrangement for minimizing the "offsetting" problem is to provide a fuser member, for example, a fuser roll or belt structure, with an outer surface or layer of silicone rubber which due to its elastic properties during operation, prevents toner offset thereto. However, critical pressure means applying a meshing force between the heated member and the backup member must be used in this arrangement as disclosed in U.S. Pat. No. 3,666,247 where it is disclosed that it is not necessary for the heated member in contact with the powdered image to carry an offset preventing liquid on its surface when the pressure means for applying a meshing force between the members provides a pressure sufficiently high to cause the coating thereon to be in a compressed state about the powder image and to deform the second member so as to create a contact zone, the pressure being sufficiently low however, to avoid detrimental effects to the substrate. Tedious pressure adjustments required for this method are undesirable, and the manufacture and maintenance of such pressure equipment is costly.
While it may be desirable to avoid completely the employment of an offset preventing liquid upon the fuser surface, from the standpoint of overall cost of operation of apparatus employing a contact fuser, a fuser utilizing a conformable surface with silicone oil continuously applied thereto has been found to yield superior results in preventing toner offset accompanied by a considerable cost savings and very wide latitude in operation. Systems are known which apply silicone oil to a silicone rubber surface, however, many of these apply the silicone oil to the surface in an intermittent fashion which is suitable for only low volume and light density image applications. Other fusing systems and devices are known which continuously apply silicone oil or other offset preventing liquid to a silicone rubber surface or other release surface, however, such systems supply the offset preventing liquid or fluid from supply reservoirs which are designed to hold a supply of the liquid or fluid, or from fuser surfaces which provide a supply of lubricant to the surface thereof by capillary action and other phenomena. Examples of these fusing devices and techniques are found in U.S. Pat. No. 3,799,401 where lubricant is conveyed from a supply reservoir to an annular absorbent member and a roller in the dispensing reservoir partially immersed in the lubricant dispenses the lubricant from the reservoir to the absorbent member which in turn supplies the lubricant to the fuser member. Other wicking devices are disclosed in U.S. Pat. No. 3,718,116 and U.S. Pat. No. 3,745,972. In U.S. Pat. No. 3,268,351, a xerographic fixing method and apparatus is described wherein an offset preventing liquid, silicone oil, is supplied to one of two Teflon coated, heated members from a roll applicator partially immersed in a silicone oil bath or reservoir. Although these systems minimize "off setting", the constant application of the fluid requires a separate fluid reservoir and fluid applying means as well as metering and control to maintain the proper amount of fluid on the roll.
Another type of roll for pressure fusing electrostatic images at elevated temperatures comprises a fuser roll having a working surface of cured silicone rubber on the cylinder, the fuser roll coating itself having a silicone oil impregnated in the working surface. Offset preventing liquid, silicone oil, can be supplied to the surface of this roll as described in U.S. Pat. No. 3,731,358 in quantities relative to the thickness of the surface impregnated with the silicone oil. Thus, when the surface of the roll can be relatively thick, for example, when external sources of heating are used, the supply of silicone oil or other offset preventing liquid may be sufficient to fuse a substantial number of toned images upon substrates, however, when an internal heating system is utilized to provide the necessary heat at the surface of the fuser roll to fuse the toner image to the substrate material, very thin coatings of offset preventing material, for example, silicone rubber, are required to provide a minimal thermal barrier at the surface of the fuser roll or member. Thus, when such a thin working surface is available, inherently provided offset preventing liquid or silicone oil in the fuser member itself will provide only a limited release life of the fuser surface, making frequent replacement of the fuser roll necessary. Frequent replacement of such a fuser roll increases servicing of the equipment and thereby increases operating costs.